1. Field Of The Invention
This invention is directed to a high density magnetic film memory storage device, and more particularly to such a memory storage device wherein the individual storage elements are patterned on a substrate using the intrinsic and structural anisotropies of the particular magnetic film, such that fringing fields about each storage element are eliminated.
2. Background Description
The first commercial random access nonvolatile magnetic memories were magnetic core systems in which circular magnetic ferrite cores were used as memory element, were arranged in large 3-dimensional arrays, and were coupled to selected wire wraps for reading and writing. These first magnetic arrays were physically large, required large amounts of power and consequently were characterized by high heat dissipation, and were very expensive to fabricate.
With the current desire to increase the information which can be processed on computer chips is the need to reduce the size of the associated memory storage space. Current semiconductor storage elements, e.g. RAM's, require a power source to retain their memory (volatile). Semiconductor memory elements will not work when the density is increased beyond a certain point because they depend upon having a sufficient amount of electrical charge per memory element, which amount becomes insufficient at a certain size. Thus when the density is too high, the amount of electrical charge in each element is too small. In contrast, the density of magnetic film storage elements can be much higher than semiconductor memory elements. Furthermore, semiconductor storage devices are not radiation hard and are subject to electromagnetic pulses.
High density magnetic film memory storage devices, such as bubble memory, on the other hand do not require a power source to retain their memory (nonvolatile). However, bubble memories are susceptible to slow-switching rates, high temperature dependence, and low stability of the bubble position. Therefore, the nonvolatility of bubble memories is unreliable as a practical matter and the current systems and methodologies used for practical utilization of bubble memories are complex and often difficult to integrate into the more conventional circuitry of present computer and memory systems.
The use of thin magnetic films of permalloy to form memory elements is described in Magnetic Materials And Their Applications, by Dr-Ing et al. pp. 623-632 (1974). The procedure described uses substrates of glass having a film of permalloy evaporated thereon. Lithography is then used to leave small spots of magnetic material, and it was learned that, if during the evaporation process, a large magnetic field is appied, the easy axis of all the spots will be aligned in a particular direction. The problem with this procedure is that every spot has a fringing field due to "open" magnetic flux, thus unwanted coupling will occur with the result that the elements can not be packed as closely as is desirable. When the spots are moved further apart, to avoid the negative affect of the fringing fields, the density to which the memory elements cam be packed will decrease. Also, after one spot has its magnetic field aligned, it will switch after a few weeks, by itself. This phenomenon is called "creep".